1. Field of the Invention
The present invention relates to a data transmission system and method, and in particular to a digital data transmission system and method for transmitting data between an intra-office subscriber line terminal equipment (the term `subscriber line terminal equipment` is referred to as SLT, hereinafter) and a network channel terminating equipment (the term `network channel terminating equipment` is referred to as NCTE, hereinafter) via a subscriber line. The SLT has a function to terminate the subscriber line connected to the NCTE. A first NCTE is provided in a subscriber's house and a first SLT is provided in a office of a vendor of a relevant public telecommunication network. The first SLT is coupled to a second SLT via the public telecommunication network, the second SLT being connected to a second NCTE, so that a first subscriber can communicate data to a second subscriber via the first NCTE, first SLT, public telecommunication network, second SLT and second NCTE.
In general, a subscriber's terminal equipment, for example, a cash dispenser in a bank, is connected with a NCTE, a subscriber, a customer of the bank in the example, directly communicating with the terminal equipment so as to input information to/take information from the NCTE. The NCTE has a function to convert signals coming from the terminal equipment into those desired by a relevant public telecommunication network. In the example of the bank, the NCTE is coupled with a host computer provided in a head office of the bank via the public telecommunication network.
Such digital data transmission systems using NCTEs have been used by various types of users and thus it has been demanded that various data transfer rates be applied in the transmission systems. Accordingly, it is necessary that NCTEs acting as a part of the transmission systems satisfy such users' demands. Further, interfaces for subscriber lines have been standardized. Thus, users have been able to prepare NCTEs freely without directly asking a vendor of a relevant telecommunication network, although previously subscribers always needed to either borrow or buy NCTEs from such a vendor in a conventional way. However, if it is assumed that a subscriber line is initially established between a subscriber's house and a vendor's office, a relevant vendor first prepares a vendor's NCTE so as to carry out tests to confirm establishment of the subscriber line.
After the tests, the subscriber may remove the vendor's NCTE from the subscriber line or switch off the power of the NCTE without informing the vendor of the removal or power switching off so as to connect another NCTE prepared by the subscriber. An SLT such as that described above provided in an office of a vendor has, in general, a function of issuing an alarm to a managing operator present in the office as a result of the SLT detecting troubles occurring along a subscriber line connecting the SLT with a relevant NCTE. Therefore, if the subscriber removes the vendor's NCTE from the subscriber line or switches off the NCTE's power as mentioned above, since the removal or power switching off results in disconnection of a circuit relevant to the subscriber line or lack of a normal response from the NCTE, an SLT terminating the subscriber line in a vendor's office detects the disconnection of the circuit or the lack of a normal response and thus issues an alarm to the managing operator present in the vendor's office. However, the alarm is not an alarm resulting from an actual trouble which the managing operator must handle but an alarm resulting from the subscriber's arbitrary behavior which is to be handled only by the subscriber itself. Such a kind of alarm is troublesome for the managing operator since the managing operator cannot find out whether the alarm is an alarm which must be handled by the operator or an alarm that does not have to be handled by the operator unless the operator directly communicates with the subscriber to inquire whether the subscriber has removed the NCTE from the subscriber line or switched off the power of the NCTE.
2. Prior Art
In order to prevent such a troublesome an alarm from being issued, the managing operator checks beforehand a condition regarding the NCTE currently connected to the subscriber line so that the operator may switch off the power of an equipment for performing the alarm issuing function in advance if it is found that the subscriber plans to remove the NCTE from the subscriber line or switch off the power of the NCTE. Thus, issuing an alarm is inhibited. Then, after the operator has confirmed that the subscriber has completed replacement of the originally connected NCTE with another NCTE, the operator switches on the power of the equipment for performing the alarm issuing function.
However, such a method by which the operator must switch off/on the equipment for issuing the alarm after the operator finds out that the subscriber's arbitrary behavior has affected the NCTE's connection with the subscriber line is still troublesome for the operator. Further, if the operator erroneously omits to switch off the power of the alarm equipment, the above troublesome alarm is issued. On the other hand, if the operator erroneously omits to switch on the power of the alarm equipment, any actual troubles do not actuate the alarm equipment to issue an alarm, so that the subscriber may be inconvenienced by these troubles.